It is generally known that the transient behaviour of proportional-integral-derivative (PID) controlled systems can be classified into a number of different categories. Depending on the amplitudes, generally referred to as the PID parameters, at which each of the proportional, integral and derivative signals are being amplified by the controller, the output of the system can reach the desired set point in a number of different manners. For example the desired set point can be reached rapidly with some degree of overshoot before settling at the desired level, the desired set point can be reached slowly substantially without overshoot, or the desired set point can be reached in a minimum time period with minimum overshoot, for example. The PID parameters or constants can be chosen in order to achieve a desired transient system behaviour or response time or dead time, and this desired behaviour can be directly related to the type of system being controlled by the PID controller. Systems whose response times are substantially constant and which are intended to be operated within operating conditions which are substantially constant can be effectively controlled by controllers with pre-configured PID constants.
As defined above, the PID parameters for some systems are configured such that a desired set point or operational output of the system is reached in a rapid manner. This can be performed however at the expense of potential overshoot and oscillation at the desired set point. In some systems this can result in a noticeable and undesired variation in the output of the system.
For example, as identified above, overshoot and oscillation about a desired set point can be eliminated by an appropriate choice of the constants for the PID controller, for example setting a high value for the derivative constant. This selection of the derivative constant has the effect of slowing down the approach of the output to the desired set point as the desired set point is reached. In this configuration, the desired set point will be obtained in a relatively slow manner. A problem with this configuration is that for a system which produces undesired and noticeable changes in output may require that the desired set point is reached in a more rapid manner.
Another way of substantially eliminating the overshoot and oscillation about a desired set point is to operate the control system using variable feedback sampling frequencies. For example, the control system can adapt the sampling frequency in accordance with desired user interface readings or in accordance with the state of the system under operating conditions. For example, the control system can increase the feedback sampling frequency during transitions between system states. For example, if the system is a luminaire, the feedback sampling frequency can be adjusted upon the dimming of the luminaire. The increased feedback sampling frequency during a transient period can provide a dynamically more stable control loop. This, however, is typically not a simple solution to the problem.
For example and having specific regard to the use of PID controllers with solid state lighting, a specific problem occurs when controlling the output of the luminaire at low intensity levels, whether it be a change in colour, colour temperature or a change in intensity. The widespread use of digital control techniques in solid-state lighting systems means that many such lights have a limited number of intensity levels, such as 28 (i.e., 256 intensity levels), for example. When changing the intensity, or controlling the intensity of a luminaire at relatively high light outputs, overshoot beyond, or oscillation about the desired set point by a few intensity steps is typically not perceivable to the eye. However, when the desired set point is at the low end of the available intensity range, overshoot beyond, or oscillation about the desired set point by only one or two intensity steps can correspond to perceptible intensity changes, possibly in the range of about 10%. This degree of overshoot and oscillation about a desired set point is typically readily perceivable and can be annoying to some viewers.
As is known, the perceived brightness of LEDs has a non-linear relationship to the radiometric intensities of the LEDs, including for example the Helmholtz-Kohlrausch effect and Bezold-Brücke phenomena. This relationship between perceived brightness and measured luminous intensity is described by, for example, Wyszecki, G., and W. S. Stiles in “Color Science: Concepts and Methods, Quantitative Data and Formulae,” New York, N.Y.: Wiley-Interscience, 2000. This relationship results in a perceived non-linear brightness when using linear control parameters. The relationship between perceived brightness and measured illuminance of an object can be approximately represented by Steven's Law and is defined as follows:B=αL0.5  (1)where B is the perceived brightness, α is a scaling constant, and L is the luminance (measured in candela per square meter per steradian) of the illuminated object at a given point on its surface. If for example, square law dimming as based on Steven's law, is not employed during adjustment of the luminous flux output of a luminaire, this non-linear relationship between the perceived intensity and the radiometric brightness of a luminaire can compound the problem of perceived intensity overshoot and/or oscillation at desired set points which represent low intensity levels.
There is therefore a need for a new method and apparatus for digital control of a lighting device, for example a solid state lighting device, in which desired set points can be reached quickly and maintained substantially without perceivable overshoot or oscillation, while operating a feedback sampling loop at constant frequency.
This background information is provided to reveal information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.